1. Field of the Invention
This invention relates to a multimedia communication apparatus, which is typified by an AV (audio-visual) communication apparatus such as a visual telephone or television conference system, that is capable of multiplexed intercommunication of such multimedia information as audio information, video information and code-data information.
2. Description of the Prior Art
Communication services using ISDN lines have recently started to be put into practical use, and AV (audio-visual) services such as visual telephones and television conference systems employing such digital lines have become the focus of much interest. Service stipulations, protocol stipulations, and multimedia multiplexed-frame structure stipulations for AV services have been announced as CCITT recommendations and recommendation drafts H.320, H.242, H.221.
In H.221, the frame structure in AV services on a 64.about.1920 kbps channel is stipulated.
FIG. 1 illustrates the H.221 frame structure on the B channel of an ISDN, in which the horizontal axis indicates the bit number and the longitudinal axis the octet number. Eighty octets construct one frame. Control of frame alignment and multiframe alignment, monitoring of communication quality and control of alarm information notification are performed by a FAS (frame alignment signal) shown in FIG. 1.
FIG. 2 illustrates the bit assignment in one multiframe, which is equal to eight sub-multiframes, of the FAS (one sub-multiframe=two frames).
In FIG. 3, designation of the capability of a terminal and the bit allocation for every medium in an actual frame, as well as various other types of control and notification operations are performed by a BAS (bit allocation signal). As shown in FIG. 3, the BAS is transmitted by even-numbered frames. The corresponding error correcting bits are transmitted by odd-numbered frames.
In H.242, communication procedure such as a performance-information exchange sequence and mode changeover sequence using the BAS on the in-channel between AV terminals is stipulated. In H.320, the overall system aspects of AV services are stipulated.
Also, in intercommunication based upon multiconnections utilizing a plurality of B and H channels, frame alignment and multiframe alignment during an additional connection utilizing the FAS, as well as an alignment establishing procedure with respect to a first connection are stipulated.
When multimedia communication of video, audio and data (all user information with the exception of video and audio), etc., is performed in accordance with the above-mentioned recommendations and recommendation drafts, the information transmission speeds of the media are decided as follows: the transmission speed of the audio information is decided by designating the audio operation mode, and the transmission speed of the data information is basically decided by designating the data mode. That which remains, upon subtracting the transmission speeds of the audio information and data information from the capacity of the communication line effective in intercommunication, is the transmission speed of the video information.
Accordingly, the transmission speed of the video information cannot be designated in a definite manner and varies depending upon the designations regarding the other media. Furthermore, in a case where only a very slow speed has been allocated as the transmission speed of the video information, it is recommended that transmission of the video information be halted. However, there are no definite stipulations regarding this.
Nevertheless, in the prior art described above, the result of subtracting the transmission speeds of such media as the audio information and data information from the total transmission speed is merely allocated automatically as the transmission speed of the video information, and therefore a drawback is that the user cannot determine whether the transmission speed of the video information is adequate for transmission of the video information. This may be considered to hold true similarly in a case where the transmission speed allocated to video information becomes inadequate when utilized connections are reduced owing to the occurrence of a connection malfunction at such time that a plurality of channels are being utilized. Though the video codec is capable of sending and receiving video information even at such very low transmission speeds, the video which is monitored takes on a highly unnatural appearance.
For these reasons, consideration has been given to a method in which transmission of the video information is simply turned off when the transmission speed of the video information falls below a stipulated value, as recommended in the above-mentioned recommendation draft of the CCITT. In such a case, however, a disadvantage is that the user cannot recognize that the transmission of the video information has been turned off. A further drawback is that when the transmission of the video information has been turned off, the transmission capacity of the line is wasted.
In a visual telephone wherein intercommunication is performed by multiplexing audio information and video information, etc., using a plurality of connections, the transmission rate allocated to the audio information is not constant owing to times when a hold designation is made. In addition, there are cases where the video information and data information is transmitted on separate connections at the same time that the audio information is transmitted.
Continuing to transmit video, when such transmission on separate connections is maintained when a hold is in effect, is wasteful of the line tariff. Furthermore, with regard also to the holding tone that is transmitted during hold, if, say, 16 kbps has been allocated as the present audio information regardless of the fact that it is unnecessary to transmit media other than the holding tone when hold is in effect, the holding tone will be transmitted in a very low-quality audio band of 16 kbps.